Wireless communication units, for example those operating in a cellular telephone system such as the Global System for Mobile communications (GSM), are known to use a broadcast reference frequency signal, sent on for example a Frequency Correction Channel (FCCH), to calibrate their operating (transmit/receive) frequency. The broadcast signal is generally transmitted from one or more base transceiver stations (BTSs). The wireless communication units use the frequency correction signal to synchronise their internal frequency generation circuits to a centralized timing system. The wireless communication units synchronise their operating frequency to match the system frequency, prior to entering into a communication. Such frequency correction techniques have also been adopted by a number of enhanced digital cellular telecommunication technologies, including general packet radio system (GPRS), enhanced general packet radio system (EGPRS) transceivers that cover low band GSM850, enhanced GSM (EGSM), high band digital communication system DCS1800 and personal communication system PCS1900 frequencies, as defined by the 3rd Generation Partnership Project 3GPP (previously standardised by European Telecommunication Standards Institute (ETSI)).
It is known to use Very Low Intermediate Frequency (VLIF) radio receivers within such wireless communication units. Such radio receivers provide the advantage over, for example, a traditional heterodyne architecture in that they comprise lower power consumption, and enable a high level of integration within an integrated circuit package.
However, a known problem with VLIF receivers is that, due to their low intermediate frequencies, it is not easy to remove radio interference using conventional filtering techniques. Consequently, for VLIF receivers, arranged to perform quadrature amplitude demodulation, it is important for the quadrature (I/Q) balancing of the receiver circuitry to be as accurate as possible, in order to minimise the effect of in-band interference due to blocking interferer signals.
Quadrature imbalance is created due to small differences in the tolerances of components in respective ‘I’ and ‘Q’ paths of the receiver circuitry. These small differences in the tolerances in the respective ‘I’ and ‘Q’ paths can result in a phase skew and/or gain imbalance between the two paths, resulting in a quadrature (I/Q) imbalance.
Radio frequency (RF) circuitry components are typically integrated within an RF integrated circuit package (RFIC), in order to provide close tolerance and better matching between the various RF circuitry components. This is particularly important when circuits are duplicated, for example to support independent quadrature signal manipulation. However, even close tolerances of components between quadrature circuits still causes variations in both the gain and the phase shift of signals. Consequently, it is necessary to calibrate individual devices in order to compensate for imbalance between the ‘I’ and ‘Q’ paths. Accordingly, it is known to provide measurement functionality within the RFIC, in order to measure quadrature imbalance, and thereby to enable any necessary calibration adjustments of the RF circuitry to be made. For example, gain imbalance of ‘X’ dB may be corrected by applying a compensating gain imbalance of ‘−X’ dB in the baseband, so that the net result is 0 dB gain imbalance. Similarly with a phase imbalance of ‘Yo’ a compensating phase imbalance of −Yo may be applied also in the baseband. Mechanisms to apply these compensating signals are well understood in the industry.
As will be appreciated by a skilled artisan, it is generally the case that a manufacturer of wireless communication units incorporating RF receivers uses RFICs designed and manufactured by a third party, i.e. a supplier. A problem with such manufacturing is that it is sometimes the case that the measurement functionality, to measure quadrature imbalance, does not function correctly or is not sufficiently accurate. Furthermore, some suppliers may not incorporate measurement functionality within their RFICs at all. Consequently, the manufacturer of the wireless communication units is not able to rely on the availability of such measurement functionality within an RFIC, and even when available, on the measurement functionality functioning correctly.
As previously mentioned, it is known to integrate the RF circuitry components within an RFIC. Therefore, in the case where measurement functionality is not provided within the RFIC, or when the functionality is provided, but is not functioning correctly, it is necessary to use an output from the RFIC, generally a Digital Baseband (DBB) signal, to determine the quadrature imbalance parameters.
Currently, it is known to measure quadrature imbalance and perform calibration during factory testing of devices, where a test signal is provided as an input to the RF circuitry, and the DBB output signal is measured to determine any quadrature imbalance. Calibration and correction can then be performed in order to compensate for imbalance within the quadrature demodulation circuitry as accurately as possible.
By way of example, U.S. Pat. No. 6,785,529 describes a system and method of compensating for an imbalance between the ‘I’ and ‘Q’ paths of a low intermediate frequency or zero intermediate frequency receiver. The method described utilises a single frequency signal as a test signal to measure gain/phase imbalance between ‘I’ and ‘Q’ paths. The test signal is generated using a signal generator in a laboratory or factory environment.
It is becoming increasingly desirable to minimise factory testing in order to minimise costs. Accordingly, the need to perform quadrature imbalance measurements and calibration during factory testing is undesirable. However, without such measurements and calibration techniques, accurate quadrature balancing cannot be reliably provided, and as such performance of the RF circuitry is likely to be significantly affected.
Thus, a need exists for an improved integrated circuit, wireless communication unit and method for determining quadrature imbalance.